The pollution being referred to here is not literal, but religious. The association of death, especially violent death, murder or suicide, was thought to pollute the area where it had taken place, as well as any objects involved (such as a noose). Pollution repelled the gods, which is why nothing associated with death (or birth) was allowed in sanctuaries. To expel the pollution, the area needed to be ritually purified; this might be achieved through fire (burning the pollution away), or with pure water (such as washing hands before a libation or sacrifice to the gods, or before entering a sanctuary).